1. Field of the Invention
The present invention relates to a toner used for electrophotographic image formation, for example image formation with a copier, a printer, a facsimile, etc., electrostatic printing, or electrostatic recording.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus, an electrostatic recording apparatus, etc., an electric or magnetic latent image is visualized using toner. For example, in an electrophotographic method, an electrostatic image (latent image) is formed on a photoconductor, then the latent image is developed using toner, and a toner image is thus formed. Then the toner image is generally transferred onto a recording medium such as paper and fixed by means of a process such as heating.
A toner used for developing an electrostatic image is generally in the form of colored particles including a binder resin which contains a colorant, a charge controlling agent, etc. Methods for producing the toner are broadly classified into the pulverization method and the polymerization method.
In the pulverization method, a toner composition obtained by melt-mixing and uniformly dispersing a colorant, a charge controlling agent, an offset preventing agent, etc. into a thermoplastic resin is pulverized and classified so as to produce a toner. The pulverization method makes it possible to produce a toner with properties which are favorable to some extent. However, selection of materials is limited. For instance, a toner composition obtained by melt mixing should be able to be pulverized and classified with apparatuses which can be economically used. This requires the toner composition obtained by melt-mixing to be sufficiently brittle. When such a toner composition is pulverized, particles with a wide particle size distribution are likely to be formed. In this situation, if a copied image with favorable resolution and tone properties is to be obtained, it is necessary to remove fine powder of 5 μm or less in particle diameter and coarse powder of 20 μm or greater in particle diameter by classification. Hence there is a problem in that the yield is very low. Also in the pulverization method, it is difficult to disperse the colorant, the charge controlling agent, etc. uniformly into the thermoplastic resin, and thus there is a problem in that the obtained toner is adversely affected in terms of fluidity, developing capability, durability, image quality and so forth.
Accordingly, a dissolved resin suspension method has been proposed in which a resin solution that dissolves a resin previously synthesized by polymerization reaction is dispersed into an aqueous medium in the presence of a dispersion (auxiliary) agent such as a water-soluble resin or a surfactant and a dispersion stabilizer such as fine inorganic particles or fine resin particles, the solvent is removed by heating, pressure reduction, etc., and a toner is thus obtained (refer to Japanese Patent Application Laid-Open (JP-A) Nos. 09-319144 and 2002-284881). According to the dissolved resin suspension method in these proposals, a toner with uniform toner particles can be obtained without carrying out classification.
Regarding an electrophotographic image forming apparatus, separability of toner from a heating member (hereinafter referred to also as “offset resistance”) is required in a fixing step by a contact heating method performed using a heating member such as a heat roller. The offset resistance is achieved by using a modified polyester resin in the dissolved resin suspension method (refer to Japanese Patent No. 3640918).
Now, binder resins, which occupy 70% or more of the compositions of toners, contain oil resources as raw materials in most cases, so that there are concerns over depletion of oil resources, and global warming caused by consuming large quantities of oil resources and discharging carbon dioxide into the air. Accordingly, if resins derived from plants which grow by taking in carbon dioxide in the air are used for the binder resins, carbon dioxide is generated and consumed repeatedly within an environment, which means that there is a possibility that the global warming and the depletion of oil resources may be able to be solved at the same time. Accordingly, a variety of toners containing such plant-derived resins as binder resins have been proposed. For example, Japanese Patent No. 2909873 proposes use of polylactic acid as a binder resin. However, when polylactic acid is used without any change to it, the actions of a thermoplastic resin are lessened at the time of fixation because the polylactic acid has a higher concentration of ester bonds than a polyester resin. Also, the toner hardens to a great extent, which presents problems in that there is a lack of toner pulverization capability and a decrease in productivity.
JP-A No. 09-274335 proposes a toner for development of electrostatic images, which includes a colorant, and a polyester resin obtained by dehydration polycondensation between lactic acid and a composition containing a trifunctional or higher oxycarboxylic acid. In this proposal, however, the polyester resin (obtained by the dehydration polycondensation between the hydroxyl group of the lactic acid and the carboxyl group of the trifunctional or higher oxycarboxylic acid) is in a branched or network form and thus is less soluble in solvent than straight-chain polyester resins. Moreover, inferior in sharp melting capability, the toner is problematic in terms of its poor low-temperature fixation properties.
In an attempt to improve thermal properties of a toner, there has been proposed an electrophotographic toner including a polylactic acid-based biodegradable resin and a terpene phenol copolymer (refer to JP-A No. 2001-166537). However, this proposal does not satisfy favorable low-temperature fixation properties and hot offset resistance at the same time.
Since the toners described in these prior art documents are all obtained by pulverization, there are problems of toner loss (caused by classification) and resultant toner disposal. Also, since the amount of energy required for the pulverization is relatively large, further reduction in environmental load is required.
Polylactic acid, which is a general-purpose, easily-available plant-derived resin, is synthesized by dehydration condensation of lactic acid or ring-opening polymerization of a cyclic lactide (refer to JP-A Nos. 07-33861 and 59-96123). Therefore, when a toner is produced using polylactic acid, any of the dissolved resin suspension methods described in JP-A Nos. 09-319144 and 2002-284881 and Japanese Patent No. 3640918 may be used. However, regarding polylactic acid, when poly-L-lactic acid or poly-D-lactic acid is used alone, it has such high crystallinity that it is hardly soluble in organic solvent, which makes the use of a dissolved resin suspension method difficult. Meanwhile, JP-A No. 2008-262179 discloses that poly-L-lactic acid and poly-D-lactic acid are mixed together so as to decrease crystallinity and improve their solubility in organic solvent.
However, polylactic acid contains a large number of polar groups per unit molecule, so that in the case where a toner is produced using polylactic acid(s) with reduced crystallinity, the toner is affected by moisture to a greater extent than in the case of polylactic acid(s) with high crystallinity. This leads to degradation of the heat-resistant storage stability of the toner, variation in the fluidity of the toner (caused by moisture absorption) and difficulty in controlling the charge amount. It is particularly difficult to reduce variation in charge amount under a condition which can belong to anywhere between a low-temperature, low-humidity condition and a high-temperature, high-humidity condition, and thus there are problems of unstable charge amount and image density.
To solve problems related to a toner's heat-resistant storage stability and charge amount variation, there is a known method of covering a toner's binder resin surface with fine powder of a styrene-acrylic copolymer.
For example, developers have been proposed in which the surfaces of particles of a styrene-acrylic resin as a binder resin of toner are covered with fine powder of an acrylic polymer or fine powder of a styrene-acrylic copolymer (refer to JP-A Nos. 60-186851, 60-186852 and 60-186854, and Japanese Patent Nos. 3789522 and 3289598).
Also, developers have been proposed in which the surfaces of particles of a polyester resin as a binder resin of toner are covered with fine powder of an acrylic polymer or fine powder of a styrene-acrylic copolymer (refer to JP-A Nos. 58-205161, 58-205163 and 58-205164, 2005-77603 and 2007-93809).
However, although the developers in these proposals are superior in heat-resistant storage stability, the surface of the binder resin of the toner is covered with fine resin particles, and thus sufficient low-temperature fixation properties may not be obtained. Also, there is a problem in that the fine resin particles on the surface may peel off owing to long-term agitation in a developing unit and thus the charge amount of the toner may change with time.
As described above, since there is generally a paradoxical relationship between low-temperature fixation properties and heat-resistant storage stability, it is difficult to achieve a favorable balance between them. Accordingly, for example, developers have been proposed in which surfaces of toner particles are covered with fine powder of two different types of acrylic polymers or fine powder of two different types of styrene-acrylic copolymers so as to divide functions, and thus a favorable balance between low-temperature fixation properties and heat-resistant storage stability is achieved (refer to Japanese Patent Nos. 4076716 and 4085942, and JP-A Nos. 08-262783 and 2005-55534).
Nowadays, energy saving in relation to electrophotographic copiers and printers is attracting much interest, and superior low-temperature fixation properties are being demanded. In the cases of Japanese Patent Nos. 4076716 and 4085942 and JP-A Nos. 08-262783 and 2005-55534, however, petroleum resins that are conventionally used in the field of toners, such as styrene-acrylic resins or polyester resins, are used as binder resins, and thus it is difficult to obtain sufficient low-temperature fixation properties.
As just described, in reality, a toner which is superior in low-temperature fixation properties, heat-resistant storage stability, hot offset resistance, fluidity, image density and haze value, and high in charge stability against changes of use conditions such as temperature and humidity, and which includes polylactic acid, and techniques related to the toner have not been obtained, and so further improvement and development are being demanded.